Before I comment, I just want to make it clear that by no means am I in the same league as Peter and NHL. I live in South Africa so I thought I could make some useful comments.
Depending on where you are going in Africa, having a bag that doesn't scream "Expensive Equipment" will be to your advantage.
I have a 20D with a Canon 17-85 IS and Sigma 175-500. As you can see, I'm missing quite a large range between 85 and 175. The solution - Sigma 80-200 F2.8. For safaris, the combination of the two sigma's would be perfect.